1. Go to settings in toolbar
  2. Click on “Edit Games”
  3. Click on Add-Button
  4. Set a name e.g. “No Limit Texas Holdem”
  5. Select a poker type game type (i.e. CASH for cash games)
  6. Fill in all min-buy-ins: very high < high < moderate < low < very low

For more information about min buys read here: How to change max cash game buyins?